Since optical fiber communications have become increasingly widespread, the fabrication of optical fibers has been a subject of intensive research and development. Optical fibers are normally made in a continuous process which involves drawing a thin glass strand or fiber from a partially molten glass preform and thereafter coating it with a polymer to increase its structural strength. The U.S. patent of Andrejco et al., U.S. Pat. No. 4,450,333, granted May 22, 1984, hereby incorporated herein by reference, describes in detail a furnace of the type that can be used to partially melt a glass preform to permit a fiber to be drawn.
The U.S. patent of Frazee et al., U.S. Pat. No. 4,957,526, granted Sep. 18, 1990, is directed to methods for monitoring coatings on the fiber and feeding back information for adjusting the coatings during real time as the fiber is being fabricated. The patent describes the use of a computer for controlling the fabrication process and the importance of reducing the cost and increasing the reliability of such methods.
The heat of the furnace and the rate of draw of the fiber must be in a proper balance so that the fiber can be drawn continuously under uniform conditions. This balance is normally accomplished by monitoring the tension of the fiber while drawing the fiber at a uniform rate, typically between one and ten meters per second. If the tension rises above a prescribed value, the heat of the furnace is typically raised which results in a reduced tension. Likewise, the furnace temperature is lowered in response to the tension falling below a prescribed range of values. Fiber tension may be measured by threading the fiber through three rollers or wheels and detecting the displacement of the middle roller which is a function of fiber tension.
It has been recognized that this conventional method of monitoring tension tends to weaken the fiber and may make it unsuitable for further use. Further, it cannot be used at very high draw speeds. Consequently, workers sometimes measure the fiber tension at a low draw speed and use that measurement to predict tension at an elevated draw speed, which may lead to inaccurate results.
The paper, "An On-Line Fiber Drawing Tension and Diameter Measurement Device," P. L. Chu et al., Journal of Lightwave Technology, Vol. 7, No. 2, February 1989, pp. 225-261, describes a non-contact tension monitoring device which directs unpolarized light transversely through the fiber and then uses the retardation of the scattered light from the fiber to determine the fiber tension. While the method avoids the problems associated with rollers that contact the fiber, it is difficult to implement because its accuracy is degraded by the axial movement of the fiber, residual thermal stress, and any ovality of the outer diameter of the fiber. The paper, "Non-Contact Measurement of Optical Fiber Draw Tension," C. G. Askins et al., Journal of Lightwave Technology, Vol. 9, No. 8, August 1991, pp. 945-947, and the U.S. patent of Mensah et al., U.S. Pat. No. 4,692,615, granted Sep. 8, 1987, are directed to non-contact tension monitoring which depends upon a measurement of a vibration of the fiber. The vibration may either occur naturally or may be excited by a puff of air. A complex wave-form describing the fiber vibration is taken and a Fourier transform of the wave-form is made to determine the fundamental frequency, or the natural frequency, of the vibrating fiber. From this, the tension of the fiber can be computed. Equipment for recording the waveform and computing the Fourier transform is required.
There remains a need in the industry for a non-contact tension monitoring technique which is relatively inexpensive to implement, and which provides a rapid determination of tension so that prompt adjustments in the furnace temperature can be made in response to it.